textproduct: Albany

WHAT HAS CHANGED

Lowered temperatures for Wednesday into the upper 60s to low 70s given increasing confidence that a coastal low will develop off the NJ/Long Island coast with eastern NY and western New England within the cool conveyor belt and stratiform rain to the northwest. Otherwise, changed shower/thunderstorm wording to reflected coverage (isolated to scattered) instead of probabilistic (slight chance/chance) for today and tomorrow as very limited shear and weak flow aloft will favor pulse type/short-lived convection.

KEY MESSAGES

1) Isolated to scattered and short-lived showers and thunderstorms today into tomorrow, favored in the higher terrain areas of the southwest Adirondacks, northern/eastern Catskills and southern Greens.

2) There is a slight risk (level 2 of 4) for excessive rainfall resulting in isolated to scattered flash flooding focused mainly south of I-90 into western New England Tuesday into Wednesday. This is due to moderate to high confidence for widespread periods of rain and a chance of thunderstorms across eastern NY and western New England from to a slow moving coastal storm.

DISCUSSION

KEY MESSAGE 1...

Surface high pressure remains positioned over Nova Scotia today with weak troughing building into the Northeast today from the Great Lakes. Temperatures and dew points will be very similar to yesterday with morning sun mixing with afternoon clouds as we reach our convective temperatures. The insolation combined with dew points in the upper 50s to around 60 and cooling temperatures aloft under the incoming trough will help generate weak instability; however, the latest HREF continues to a low chance (under 30-40%) for MU CAPE to exceed 1000 J/kg late this afternoon favored in the higher terrain areas. Given weak forcing and very weak shear aloft (15kts or less), expecting pulse-type isolated to widely scattered convection favored in the southwest Adirondacks, northern/eastern Catskills, southern Greens, and Taconics that will be short-lived as downdrafts from any showers/storms quickly choke off updrafts. While PWATs over 1" can support brief heavy downpours, the brief duration and very weak shear will mitigate any flooding or severe weather concerns.

Most showers/storms will be diurnally driven and diminish with the loss of daytime heating; however, a few isolated showers may lingering overnight as shown in the latest CAMs. The weak trough from today slides into New England tomorrow but CAMs still suggest another day with pulse-type and diurnally driven convection given westerly flow aloft advecting a slightly warmer/more humid air mass into the region resulting in slightly higher instability (30 to 60% chance for at least 1000 J/kg of SB CAPE). Lapse rates are still weak (under 6C/km) under broad troughing aloft but CAMs show greater coverage of diurnally driven pulse-type isolated to scattered convection, again favored in the higher terrain areas. Weak shear will mitigate any flooding/severe concerns but there is a low to moderate chance for brief downpours within any storm.

KEY MESSAGE 2...

There is moderate to high confidence in a coastal low developing off the NJ/southern New England coast for Tuesday into Wednesday resulting in periods of widespread rain tracking into eastern NY and western New England. The guidance over the last two model cycles has come into better agreement with the EC converging towards to the GFS/CMC solution which have shown strong upper level divergence ahead of an amplifying parent shortwave from the Great Lakes resulting in a coastal low south of Long Island. However, there remains uncertainty in how quickly the parent trough closes off aloft which will determine how far north the ensuing coastal low forms. The EC remains the most aggressive with the parent trough closing off the earliest resulting in the coastal low developing further north closer to NYC/Long Island. This would allow the stratiform rain band within the deformation zone on the north side of the low to track well into eastern NY and western New England. However, the GFS and CMC are slightly delayed in the parent trough maturing/closing off allowing the coastal to form further south off the NJ coast, keeping the heaviest rain mainly south of I-90.

The exact track of the low will determine where the heaviest rainfall amounts occurr and greatest chance for flooding concern but the increasing agreement on the overall synoptic evolution gives us more confidence in messaging a stratiform, moderate rain event. There is good agreement in the warm conveyor belt taping into Gulf and western Atlantic moisture with PWATs nearing 2" wrapping into the low as it pivots/slows down on Wednesday. The moderate rainfall within the deformation zone (and eventual TROWAL) likely also pivots as the sfc low deepens and matures heading into coastal New England. This raises concerns for isolated to scattered flash flooding as this region can favor efficient, warm rain processes producing heavy rainfall rates. Latest flash flood guidance shows it will take more than 2 to 3" of rain within 3 hours to raise flash flooding concerns thanks to dry antecedent conditions but a pivoting band within a moisture rich environment with weak instability raises some concerns. WPC maintains its slight risk in the latest Day 3/4 Excessive Rainfall Outlook focused in western New England and areas mainly south of I-90 where the highest probabilities for greater than 2" of rain (40-50%) within the 24hr period between 8PM Tues and 8PM Wed exist. Easterly winds up to 850hPa shows there will likely be terrain enhancement in the eastern Catskills and eastern slopes of the southern Greens/Berkshires/Taconics where the highest probabilities are for exceeding 2" in 24 hrs. There may even be some downsloping impacts on the west side of the Taconics into portions of the Capital District leading to lower rainfall amounts. Given the track uncertainties, there is still a large spread in total QPF amounts between the 25th and 75th percentile values but latest probabilities show at least a 60% chance regionwide for 1" or more of rain suggesting moderate to high confidence in a soaking event for most of eastern NY/western New England starting Tues PM/evening lasting into Wed night. Lowest chances remain in the southwest Adirondacks which may be too far north/west of the coastal low and heavy rain band. If confidence in isolated to scattered flash flooding increase, a targeted Flood Watch for flash flooding (river flooding is not expected) may be needed but we will continue to watch trends and collaborate with neighboring WFOs.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

Through 18z/Mon...VFR conditions are expected to continue for much of the upcoming TAF period with some cumulus clouds and passing cirrus clouds at times. Any showers through this period will likely remain west of the TAF sites. If there are enough breaks in the clouds, patchy fog may form at KGFL/KPSF between 08-11z/Mon where brief vsby reductions to at least IFR are possible. Winds will be mainly light at 5 kt or less through the period mainly from the southeast to southwesterly directions.

Outlook...

Monday Night: Low Operational Impact. NO SIG WX. Tuesday: High Operational Impact. Definite SHRA...TSRA. Tuesday Night: High Operational Impact. Definite SHRA...TSRA. Wednesday: High Operational Impact. Definite SHRA...TSRA. Wednesday Night: High Operational Impact. Likely SHRA...TSRA. Thursday: High Operational Impact. Likely SHRA...TSRA. Thursday Night: Low Operational Impact. Slight Chance of SHRA. Friday: Low Operational Impact. NO SIG WX.

ALY WATCHES/WARNINGS/ADVISORIES

CT...None. NY...None. MA...None. VT...None.